1. The Field of the Invention
The present invention relates to methods of forming a silicon-containing structure over a semiconductor substrate. More particularly, the present invention relates to a method of forming a multilayer structure having successively a first layer of silicon-containing material, a relatively thin layer of silicon dioxide, and a second layer of silicon-containing material.
2. The Relevant Technology
Integrated circuits are currently manufactured by an elaborate process in which semiconductor devices, insulating films, and patterned conducting films are sequentially constructed in a predetermined arrangement on a semiconductor substrate. In the context of this document, the term xe2x80x9csemiconductor substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term xe2x80x9csubstratexe2x80x9d refers to any supporting structure including but not limited to the semiconductor substrates described above. The conventional semiconductor devices which are formed on the semiconductor wafer include capacitors, resistors, transistors, diodes, and the like. In advanced manufacturing of integrated circuits, hundreds of thousands of these semiconductor devices are formed on a single semiconductor substrate.
Integrated circuit manufacturing often requires formation of one layer of silicon-containing material over another layer of silicon-containing material. For convenience, an underlying silicon-containing layer will be referred to hereinafter as the first layer and an overlying silicon-containing layer will be referred to as the second layer. Frequently, the properties of a first layer will differ from those of a second layer in one or more important ways. For instance, the two layers may have different concentrations of dopants, thereby giving each layer different electrically conductive properties. The two layers may also have different crystalline structures, such as monocrystalline, polycrystalline, amorphous, cylindrical grain polysilicon, hemispherical grain, and spherical grain.
It is well understood that grain boundaries in highly doped, highly crystalline polysilicon facilitate outgassing and migration of dopants. It is surmised that diffusion of dopants is especially problematic when the dopants move into an adjacent second layer of silicon-containing material, thereby creating nonuniformities in the second layer. For example, a sheet resistance measurement taken at any particular point on the second layer may significantly depart from the average sheet resistance of the second layer. Dopant diffusion into adjacent layers may also cause a structure that contains the layers to having a nonuniform thickness.
It has been found that a silicon dioxide layer formed on the surface of a highly doped, highly crystalline polysilicon layer acts as diffusion barrier against outgassing of dopants. However, a layer of silicon dioxide can have undesirable consequences on a structure in which it is used. Techniques for forming a layer of silicon dioxide typically involve exposing a silicon layer to a thermal oxidation process, thereby heating not only the surface of the polysilicon layer, but also the semiconductor surface on which it lies. This heating sometimes induces diffusion of dopant material from active regions in the semiconductor substrate into adjacent previously undoped regions. Enlargement of active regions can cause inefficiency or failure of semiconductor devices. Moreover, conventional methods of forming silicon dioxide diffusion barriers produce silicon dioxide layers sufficiently thick to impair the conductivity of structures in which they are used.
What is needed is a method of forming an oxide layer upon a layer of doped silicon-containing material that adequately prevents diffusion and outgassing of dopants. A method is needed for forming an oxide layer sufficiently thin so as to not significantly reduce the conductivity of the structure in which it is used. It would also be advantageous to provide a method for forming an oxide layer that does not inherently expose nearby active regions of a semiconductor substrate to high temperatures.
The present invention relates to a process for forming a multilayer structure having successively a first layer of silicon-containing material, a relatively thin layer of silicon dioxide, and a second layer of silicon-containing material. Each of the first layer and the second layer of silicon-containing material is substantially composed of materials selected from the group consisting of monocrystalline silicon, polysilicon, amorphous silicon, spherical grain (BSG) polysilicon, and hemispherical grain (HSG) polysilicon. The oxide layer has a substantially uniform thickness in a range from about 1 Angstrom to about 20 Angstroms and consists essentially of silicon dioxide.
In one embodiment of the process, a first layer is formed over a semiconductor substrate. The first layer contains dopants, preferably in a high concentration in a range from about 5 xc3x971019 atoms/cm to about 1 xc3x971021 atoms/cm3. An oxidation process is conducted on the first layer to form an oxide layer having a thickness between about 1 Angstrom and about 20 Angstroms. The preferred oxidation process uses an aqueous bath with an oxidizing agent bubbled therethrough. The aqueous bath preferably has a temperature in a range from about 0xc2x0 C. to about 200xc2x0 C. Finally, a second layer of silicon-containing material is formed upon the oxide layer. The oxide layer substantially prevents diffusion of dopants out of the first layer and into the second layer. It is surmised that the second layer is thereby protected from dopants that would otherwise diffuse thereinto and cause nonuniformities of various properties, including sheet resistivity and thickness. Preferably, the first layer, the oxide layer, and the second layer form a laminate having an average sheet resistance and having a plurality of points thereon, where the sheet resistance at each point is within 10% of said average sheet resistance. Alternatively, it is preferable that the second layer has a plurality of points thereon and has an average thickness, where the thickness at each point on the second layer is within 7% of the average thickness of the second layer.
In a second application of the invention, a first layer of silicon-containing material is positioned over a semiconductor substrate. A contact opening is cut through the first layer to extend to a contact surface on the semiconductor substrate. The first layer has an exposed surface including both of a sidewall that partially defines the contact opening and a top surface. An oxidation process is used to form an oxide layer on the exposed surface of the first layer. A second layer of silicon-containing material is formed upon the oxide layer, extending into the contact opening and to the contact surface. Again, the oxide layer substantially prevents diffusion of dopants out of the first layer and into the second layer.